1. Field of the Invention
The present invention relates to a current control circuit including a current detection circuit and performing control in response to detected current. The present invention relates to, for instance, a technique effectively used in a charging control IC (semiconductor integrated circuit) on which a charging control circuit is mounted for charging a secondary battery.
2. Description of Related Art
A charging device of a secondary battery includes an IC on which a charging control circuit is mounted to control charging current. The charging current is controlled by a current control transistor composed of a MOSFET (insulated gate field effect transistor, hereinafter referred to as MOS transistor) provided between an input terminal to which a DC voltage is input from a primary power source, such as an AC adaptor, and an output terminal to which a secondary battery is connected.
In such a conventional charging control IC, a current flowing through the current control transistor is detected during a charging operation to control the charging current to be constant. A known method of detecting the charging current in such a constant current control mode is that a sense resistor for current detection is connected in series to the current control transistor to detect the current based on the voltage drop in the resistor Although the method relatively precisely detects the current, a large current flowing through the sense resistor causes a large power loss in the sense resistor, thus reducing the power efficiency.
To address the circumstance, the following method of detecting current has been proposed: a smaller transistor is provided in parallel to the current control transistor; the gate voltage identical to that of the current control transistor is applied to generate a current reduced in proportion to the charging current in a current mirror circuit; the generated current is applied to the sense resistor to detect current based on the voltage drop in the resistor. The method enhances the power efficiency because only a small current flows through the sense resistor. Due to fluctuation in load, however, bias conditions of a current detection transistor are different from those of the current control resistor. Thus, a current reduced precisely in proportion to the charging current cannot be provided, resulting in low detection accuracy.
As shown in FIG. 6, Japanese Unexamined Patent Application Publication No. 2009-294981 discloses an invention relating to a circuit that includes a bias control transistor Q3 disposed in series to a current detection transistor Q2 which is current-mirror connected to a current control transistor Q1 and a differential amplifier AMP1 receiving the drain voltages of the current control transistor Q1 and the current detection transistor Q2 as an input and having an output terminal connected to a gate terminal of the bias control transistor Q3. The virtual short of the differential amplifier provides bias conditions of the current detection transistor Q2 identical to those of the current control transistor Q1, thus enhancing current detection accuracy.
No problem arises in the charging control circuit disclosed in Japanese Unexamined Patent Application Publication No. 2009-294981 in the case of a primary power source that generates relatively stable voltage and current, such as an AC adaptor. Problems arise, however, with a primary power source having variable voltage and current, such as a solar battery. Specifically, first, in the case of an extremely small charging current due to a limited amount of solar radiation, a difference in voltage between a primary source 20 and a secondary battery 30, specifically a difference in voltage Vq1 between the source and drain of the current control transistor Q1 is very small. Thus, the charging current cannot be precisely detected due to an effect of offset voltage of the differential amplifier AMP1. Second, with a widened range of a controllable charging current, a voltage detected in a resistor Rp for current-voltage conversion is too high at a high charging current. Thus, a detected voltage Vr1 is clamped to an input voltage Vin as in a range P3 in FIG. 7B, leading to uncontrollable current.